Abstract

B-stage processing is a term used for one-pot thermosetting formulations that can be partially cured or pre-dried, after being applied to a substrate and then cured under heat and pressure. B-staging has processing advantages that can be exploited in assembly processes, especially the possibility of splitting the different steps of a process in a production line, in terms of time and location, thus helping to prevent manufacturing bottlenecks. However, in B-stage processing the pre-drying process has usually tight time and temperature constraints that make necessary a careful control to avoid over-drying leading to loss of properties such as adhesion and material waste. Alternative dual-curing procedures aim at eliminating the above drawbacks while keeping and even enhancing their processing advantages [1-3]. Further advantage comes from the possibility of tailoring the material structure and properties between both curing stages by changing the formulation composition. This represents a significant advantage over conventional B-stage, in which the material properties in the intermediate stage are kinetically controlled, depending on temperature/time and vitrification. This approach opens a new way for flexible processing, not just in terms of curing stageability but also in terms of the intermediate material characteristics (adhesion, flowing ability, rigidity,…). In addition, the latent character of the second curing stage ensures good storage stability between both curing stages.
In this work, the sequential UV/thermal processing of acrylate-epoxy clearcoats is studied. Mixtures of methacrylates/acrylates with different side chain and functionality, at least one of them bearing an epoxy group, are photocured to obtain a linear or cross-linked polymer that is subsequently post-cured at elevated temperatures to obtain the final cross-linked material. A combination of a radical photoinitiator and a cationic thermal latent initiator is used in order to permit sequential controlled curing. Different formulation compositions are examined in order to analyze the possibility of custom-tailoring the structure and properties between both curing stages and at the end of the process. The sequential UV-curing and thermal curing processes are studied with photoDSC, FTIR/ATR and DSC. The thermal-mechanical properties of the materials in the intermediate and final states are assessed with DMA, DSC and TGA. The potential application of this curing process is also illustrated.
[1] K. Studer, C. Decker, E. Beck, R. Schwalm, Eur. Polym., J. 41 (2005) 157
[2] J. A. Carioscia, J. W. Stansbury, C. N. Bowman, Polymer, 48 (2007) 1526
[3] C. F. Carlborg, A. Vastesson, Y. Liu, W. van der Wijngaart, M. Johansson, T. Haraldsson, J. Polym. Sci. A: Polym. Chem., 52 (2014) 2604